* When signing multiple documents (if 'Allow only one document' is disable)
  then 'Save as attachment' is not applied and signed result is not
  saved as attachment.
* Add tests.
* Why not taking the occasion to add the whole configuration at report level
  (if to be signed or not, the domain, etc...)?
  See https://github.com/OCA/reporting-engine/pull/533#issuecomment-898321161
